Femoral Nerve

Anatomy

The femoral nerve (▶ Fig. 7.1, ▶ Fig. 7.2, ▶ Fig. 7.3, ▶ Fig. 7.4), the largest branch of the lumbar plexus, runs laterally and caudally through the psoas major muscle and anterior to the iliacus muscle (the latter of which it supplies) at a mean sagittal angle of approximately 27.78 degrees. ¹^, ²^, ³^, ⁴^, ⁵ It originates from the posterior divisions of the ventral rami of the second, third, and fourth lumbar spinal nerves within the psoas major muscle (~ 9 cm from the skin surface) and runs between the lateral femoral cutaneous nerve and the obturator nerve from the lumbar plexus. ⁶^, ⁷^, ⁸ The femoral nerve emerges from the lateral border of the psoas major muscle 4 cm superior to the inguinal ligament but still beneath the fascia of the iliacus muscle. ⁶ It enters the thigh with the iliacus muscle while passing under the inguinal ligament and through the lateral space of the lacuna musculorum, approximately half the medial–lateral distance between the anterior superior iliac spine and the pubic symphysis. ⁴^, ⁵^, ⁶ It enters the thigh lateral to the femoral artery and therefore outside the femoral sheath. ¹^, ⁶ The femoral nerve emerges from under cover of the inguinal ligament about half-way between the anterior superior iliac spine and the pubic tubercle. The femoral neurovascular bundle is contained within the femoral triangle at the level of the inguinal crease, where it is covered by layers of skin, fat, and fascia lata. ⁹ The femoral triangle is formed laterally by the sartorius muscle, medially by the adductor longus muscles, and superiorly by the inguinal ligament. ⁹ Approximately 1.50 ± 0.47 cm distal to the inguinal ligament (others have reported a distance of 3–5 cm), before the femoral crease, the femoral nerve splits into numerous posterior and anterior branches, which are observed in 80% of cases to be divided by the lateral circumflex femoral artery. ⁴^, ⁶^, ⁷^, ¹⁰^, ¹¹ One anatomical study showed that most cadavers (83.3%) possessed a femoral nerve located anterior to the profunda femoris artery, while the femoral nerves in the remainder were posterior to this structure (16.7%). ¹

Fig. 7.1 Schematic drawing of the course of the femoral nerve and the muscles it innervates.

(Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM, Schuenke M, Schulte E, Schumacher U. Atlas of Anatomy. 2nd ed. New York, NY: Thieme Medical Publishers; 2012; ▒Fig. 29.16░. Illustration by Karl Wesker.)

Fig. 7.2 Course of the femoral nerve under inguinal ligament and into the thigh. Note its position lateral to the femoral sheath and superficial to the iliacus muscle.

Fig. 7.3 Dissection of the femoral nerve in the thigh and noting its long saphenous branch.

Fig. 7.4 Branching pattern of the (a) femoral nerve and its (b) fascicular patterns.

The nerve receives its blood supply from the iliolumbar artery when travelling through the pelvis, the deep circumflex iliac artery when traversing the inguinal region, and the lateral circumflex femoral artery upon entering the thigh. ⁷ Interestingly, some have discovered that the deep circumflex iliac artery on the left side gives fewer branches than its equivalent on the right, potentially rendering the femoral nerve on that side more susceptible to ischemic injury. ⁷ Three different lymphatic routes are known to drain the femoral nerve. In the iliac fossa, one or two routes drain, which then move to lymph nodes near the external iliac artery. ¹² The third lymphatic route drains into the lymph nodes near the femoral canal after leaving the nerve where the femoral nerve passes under the inguinal ligament. ¹²

The femoral nerve displays multiple subtle changes in its structure and relationship with other structures as it descends into the leg. It is nearly 50% wider and significantly closer to the fascia lata (6.8 vs. 26.4 mm) at its location in the inguinal crease than at the inguinal ligament. ⁹ It is partially covered by the femoral artery at the level of the inguinal crease in approximately 70% of cases, but in only 11.7% of cases in the inguinal ligament region. ⁹ Interestingly, the spatial arrangement within the femoral neurovascular bundle seems more consistent at the level of the inguinal crease (where the nerve is also more superficial) than at the level of the inguinal ligament. ⁹ Ultrasound imaging has shown that as the nerve travels distally, it becomes generally flatter, wider, and less compact. ¹¹ Below the inguinal ligament, the branch-free length of the nerve is approximately 1.0 to 1.5 cm. ⁶ Cross-sections of the compound femoral nerve (i.e., the portion of the nerve between the inguinal ligament and the first branch formed in the thigh) are oval, with average major and minor diameters of approximately 10.5 and 2.3 mm, respectively. ⁶ Closer to the lumbar plexus, before the flattening of the nerve, the thickness is between approximately 4.52 and 4.85 mm. ² The medial cutaneous branches and nerves of the vastus medialis, vastus intermedius, vastus lateralis, and rectus femoris were dissected and found to be arranged in that order from a medial to lateral direction. ⁶ Within the femoral nerve, a muscular branch to the sartorius muscle and two sensory branches (the intermediate and medial cutaneous branches of the thigh) make up the anterior division, while the posterior division comprises the motor nerves to the quadriceps femoris and articularis genus, and the saphenous nerve. ⁷ In the thigh, the most medial branch is that to the pectineus muscle, some authors contending that this is the most anterior portion of the nerve and others suggesting that the branch to the sartorius muscle holds this distinction. ⁶ One other point of contention is that while some claim that the medial cutaneous nerve and adductor longus branch of the femoral nerve are the first to branch away in the thigh, others claim that the medial cutaneous nerve branches more distal to the pectineus and sartorius innervating branches. ⁶ Some variations in these structures may be due to the capacity of the obturator nerve to give rise to several of these muscle-bound branches. ⁶

Another study showed the femoral nerve was found to be made up of three-layered intrinsic divisions sorted from superficial to deep. ¹³ The first division was an intrinsic structure in the femoral nerve. The middle layer or second division was predominately connected to the sartorius muscle. The deepest layer or third division included the saphenous nerve and muscular branches to the quadriceps femoris. The most superficial layer contained the cutaneous branches crossing over the proximal part of the sartorius muscle. These often communicated with the lateral femoral cutaneous nerve with some moving out toward the lateral aspect of the thigh instead of the main trunk of the lateral femoral cutaneous nerve. These branches were always the most superficial layer of the femoral nerve before it divided into several branches as it passed under the inguinal ligament. These cutaneous branches usually divided containing both cutaneous branches and muscular branches. The cutaneous branches penetrated the sartorius muscle. The muscular branches that supplied the sartorius muscle were in the middle layer of the femoral nerve. The most superficial layer also contained muscular branches going to the pectineus muscle and cutaneous branches headed toward the medial aspect of the thigh. These cutaneous branches showed a close correlation with the lateral femoral cutaneous nerve. ¹³

One key facet of the fascicular anatomy of the femoral nerve is branches with individual fascicles or groups of fascicles within the compound femoral nerve can be traced to their branching points and into the subsequently formed nerves (▶ Fig. 7.4). ⁶ Such branches with their own traceable fascicles include those to the sartorius, pectineus, vastus medialis, vastus intermedius, vastus lateralis, and rectus femoris muscles, as well as the medial cutaneous and saphenous nerves. ⁶ Fascicles that formed the branches to the vastus medialis, vastus intermedius, and vastus lateralis distally were consistently observed to be in the center and dorsally within the proximal femoral nerve. ⁶ Fascicles for the sensory nerves, such as the saphenous and medial cutaneous, as well as the nerve innervating the rectus femoris, were consistently located on the periphery. ⁶ Whereas the fascicles making up the sartorius nerve generally branch away in a ventral fashion, they are often located in the lateral, medial, or central portions of the nerve. ⁶ Fascicles that become the pectineus nerve are often located in a ventral position. ⁶ Few fascicular anastomoses are seen between the distal nerve branches and the proximal compound femoral nerve, and no interfascicular plexus are formed either. ⁶

Tracing the femoral nerve to the various structures it innervates should indicate the purpose of these branches. The muscles that the femoral nerve innervates are primarily responsible for leg extension at the knee joint and flexion of the thigh, ultimately making this nerve of key importance in standing up (for which the three vasti muscles are primarily designed) and stepping. ⁶ Other muscles innervated by this nerve include the biarticulate rectus femoris and sartorius, which are involved in the motion needed to progress from sitting to standing. ⁶

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